The invention relates to electrical circuit arrangements involving the transmission and reception of electromagnetic radiation, and is more specifically concerned with avoiding the effects at a receiver of unwanted interference between the outputs of two transmitters. The invention may, for example, be applied to intruder detection systems of the microwave radiation type in which microwave radiation is radiated into an area or areas under surveillance by a number of different microwave transmitters, and the effect on such radiation of an intruder is monitored by processing the radiation received by one or more microwave receivers. In one specific example of such an intruder detection system using a plurality of microwave transmitter/receiver units, each has an antenna which radiates microwave radiation, such as in pulsed form, into the area under surveillance and which receives radiation reflected back by objects in the area under surveillance. Each unit compares the transmitted and reflected radiation so as to detect the presence of an intruder by the Doppler shift caused to the frequency of the reflected radiation by the intruder's movement. In such systems, it is important that there should be no confusion between signals caused by movement of an intruder and signals caused by the effect, at a particular antenna, of the radiation received not by reflection of the radiation emitted by that antenna but received from the antenna of another transmitter/receiver unit. Radiation may be received from another transmitter/receiver unit either directly or by reflection such as by reflection from some stationary object within the area under surveillance. Such received radiation may cause production by that unit of a frequency difference, and in such a case, it is important that this frequency difference should not be misinterpreted as the presence of an intruder, causing a false alarm.